1. Field of the Invention
The present invention relates to a magnetoresistive device adapted to read the magnetic field intensity of magnetic recording media or the like as signals, a thin-film magnetic head comprising that magnetoresistive device, and a head gimbal assembly and a magnetic disk system, one each including that thin-film magnetic head.
2. Explanation of the Prior Art
In recent years, with an increase in the recording density of hard disks (HDDs), there have been growing demands for improvements in the performance of thin-film magnetic heads. For the thin-film magnetic head, a composite type thin-film magnetic head has been widely used, which has a structure wherein a reproducing head having a read-only magnetoresistive device (hereinafter often called the MR device for short) and a recording head having a write-only induction type magnetic device are stacked together.
With an increase in the recording density, there has been a growing demand for the reproducing device of a reproducing head to have narrower shield gaps and narrower tracks, and there is now a GMR device of the CPP (current perpendicular to plane) structure (CPP-GMR device) proposed in the art, in which upper and lower shield layers and a magnetoresistive device are connected electrically in series to make do without any insulating layer between the shields. This technology is thought of as inevitable to achieve such recording densities as exceeding 200 Gbits/in2.
Such a CPP-GMR device has a multilayer structure comprising a first ferromagnetic layer and a second ferromagnetic layer between which an electroconductive, nonmagnetic intermediate layer is sandwiched. A typical multilayer structure for the spin valve type CPP-GMR device comprises, in order from a substrate side, a lower electrode/antiferromagnetic layer/first ferromagnetic layer/electroconductive, nonmagnetic intermediate layer/second ferromagnetic layer/upper electrode stacked together in order.
The direction of magnetization of the first ferromagnetic layer that is one of the ferromagnetic layers remains fixed such that when an externally applied magnetic field is zero, it is perpendicular to the direction of magnetization of the second ferromagnetic layer. The fixation of the direction of magnetization of the first ferromagnetic layer is achieved by the exchange coupling of it with an antiferromagnetic layer provided adjacent to it, whereby unidirectional anisotropic energy (also called the “exchange bias” or “coupled magnetic field”) is applied to the first ferromagnetic layer. For this reason, the first ferromagnetic layer is also called the fixed magnetization layer. By contrast, the second ferromagnetic layer is also called the free layer. Further, if the fixed magnetization layer (the first ferromagnetic layer) is configured as a triple-layer structure of a ferromagnetic layer/nonmagnetic metal layer/ferromagnetic layer (the so-called “multilayer ferri-structure” or “synthetic pinned layer”), it is then possible to give a strong exchange coupling between two ferromagnetic layers, thereby effectively increasing the exchange coupling force from the antiferromagnetic layer, and reducing the influences on the free layer of a static magnetic field resulting from the fixed magnetization layer. Thus, the “synthetic pinned structure” is now in extensive use.
To meet the demands toward recent ultra-high recording densities, however, it is an essential requirement to diminish the “width” and “height” of the magnetoresistive device built in the reproducing (read) head.
To lower the height of the magnetoresistive device, viz., to make the device much thinner, U.S. Pat. Nos. 5,576,914, 6,724,583, 7,117,122, etc. have come up with a novel GMR device structure basically comprising a simple triple-layer structure of a ferromagnetic layer/nonmagnetic intermediate layer/ferromagnetic layer. According to those publications, under the action of a bias magnetic field, there is an initial state created in which the magnetizations of two magnetic layers are inclined about 45° with respect to the track width direction. Upon detection of a signal magnetic field from a medium in the initial state of the device, the directions of magnetization of the two magnetic layers change as if scissors cut paper, with the result that there is a change in the resistance value of the device. In the present disclosure, the GMR device of such structure may be called the “scissors type GMR device” for the sake of convenience.
One possible approach to the application of bias magnetic fields to the head structure using the aforesaid prior art “scissors type GMR device” is to place a permanent magnet 900 such as CoPt at the rear site of a device 800 as shown typically in FIG. 15. When such an arrangement is used, however, much of the magnetic flux generated from the permanent magnet 900 for the adjustment of the directions of magnetization of free layers 411 and 415 of the device 800 leaks out to the sides of upper and lower shield layers 901 and 905: the function that it should have to adjust the directions of magnetization tends to be in the wane. In other words, the permanent magnet 900 must be much larger than other parts. However, as the height of the device 800 grows low, there would be no option but to make the permanent magnet 900 thinner or lower: it would be very difficult to make sure the permanent magnet 900 has plenty of function of adjusting the directions of magnetization.
As device size grows small, it causes a decrease in the spacing between magnetic signals recorded in a magnetic recording medium, viz., bits, and likely interferences from neighboring bits would be an obstacle to the normal reading of magnetic signals.
The situations being like this, the present invention has been made for the purpose of providing a magnetoresistive device which makes it possible to adopt the structure capable of narrowing down the read gap (the gap between the upper and the lower shield) to meet recent mounting demands for ultra-high recording densities; and enables stable magnetic-field biasing layer (biasing layer) to be applied by way of a simplified structure and interferences from neighboring bits to be kept in check to make a magnetic signal profile so sharp that an effective device width can be narrowed down for the normal reading of magnetic signals, and a thin-film magnetic head comprising that magnetoresistive device as well as a head gimbal assembly and a magnetic disk system, one each comprising that magnetoresistive device.